Equipped with a pair of rapidly deployable tentacles, little will escape the squid's grip once a victim has wandered within range. Justin Shaffer from the University of North Carolina, USA, explains that a hunting squid can double the length of its lethal tentacles in as little as 15 ms, whereas the eight arms are used for slower more mundane tasks, such as eating and swimming. So how have squid modified their tentacles to contract so fast?
Shaffer explains that muscles contract when thick filaments of myosin hydrolyse ATP to slide past thin filaments of another muscle protein, actin. The smallest contractile unit is known as the sarcomere and when Bill Kier looked at the arrangement of sarcomeres in the muscle fibres in squid arms and tentacles, he realised that the short sarcomere structures in the tentacles' transverse muscle fibres could account for the muscle's rapid contraction. However, the speed of most muscular contractions is regulated by the rate at which myosin hydrolyses ATP. ‘Every other animal uses different myosin isoforms to control speed’, explains Shaffer. Suspecting that squid also use specifically tailored myosin molecules to fine-tune the speed of muscular contraction, Shaffer and Kier decided to sequence myosin mRNA – which is later translated into protein – from various squid muscle tissues to find out whether squid express specialised myosins in their fast-contracting tentacles (p. 239).
Coming from a cardiac biochemistry background, Shaffer was well prepared to begin searching for distinct myosin mRNA molecules in the tentacle, arms, mantle, fin and funnel retractor muscles of the squid, Doryteuthis pealeii. Dividing the colossal myosin transcript (about 6600 base pairs long) into six regions, cloning each region and sequencing it, Shaffer reconstructed the mRNA sequences of each myosin isoform from each tissue and was amazed to find the same three myosin transcripts turning up in all five tissues. ‘I was doing a lot of these samples side by side’, recalls Shaffer.‘We had this hypothesis that there should be different myosins and I kept getting all this sequence data showing the same sequence, whether it was in the tentacle or the mantle, and I thought, “there is no way this is right”, but I kept doing it, I ran all my controls and it kept coming out.’
So the squid were not producing a uniquely tailored high-speed myosin in the tentacle to account for its unusually fast contraction; instead they had modified the muscular structure with their short sarcomeres to catch prey fast. ‘I was surprised’, admits Shaffer. He explains that invertebrates such as lobsters and scallops produce so-called ‘fast’ myosin isoforms in combination with short sarcomeres to produce fast-contracting muscle, and says, ‘This squid is really unique because so far, it is the only animal we have encountered that uses only ultrastructural differences to alter contractile performance.’
However, Shaffer points out that even though each tissue produces all three myosin isoforms, it is possible that the isoforms are specifically produced in the different muscle fibre types (transverse vs longitudinal vs helical) that construct a muscle to fine tune contractile performance. He is also keen to investigate myosin isoform expression in the muscles of other cephalopods to find out whether they too use muscle ultrastructure modification to speed up muscle contraction.